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Atmospheric particle acidity, which is described by particle pH, has effects on aerosol formation, composition, and toxicity. It also has impacts on ecosystem and climate. During the past 15 years, NOx and SO2 emissions in the United States have decreased by approximately 56% and 82%. In the atmosphere, both NO2 and SO2 are oxidized to form HNO3 and H2SO4, respectively, which are the most abundant acidic species in aerosol. In this study, data from the Atlanta Aerosol Nucleation and Real-Time Characterization Experiment (ANARChE) in August 2002 are reanalyzed. Fast ammonia and ammonium measurements were carried out in this experiment, which makes it possible to investigate the diurnal cycle of particle pH, and pH time evolution over the past 15 years in urban Atlanta. Fine particle pH is calculated by ISORROPIA-II, a thermodynamic equilibrium model. For comparison, data from the Southeastern Aerosol Research and Characterization study (SEARCH) at the same site in August 2016 are also used for pH calculation. The result is counterintuitive that fine particle pH did not show an increasing trend, but has slightly decreased despite the emission reduction and the decrease in sulfate and nitrate levels. The average fine particle pH from ANARChE is 1.92 ± 0.58 (±SD), with a median of 1.88, while the average pH from SEARCH is 1.68 ± 0.48, with a median of 1.67. The predicted concentrations and partitioning fractions of semivolatile species (NH3-NH4+, HNO3-NO3−) agree with observations. Sensitivity tests were performed to evaluate the effects of measurement uncertainties on model predictions but no significant effects were observed. The small change in pH from 2002 to 2016 could be explained by the decrease in particle liquid water content and the volatilization of NH4+. This is supported by the decrease of observed NH4+ partitioning fraction (from 44.6% in ANARChE to 29.5% in SEARCH).